1. Field of the Invention
The present invention relates to bicycle front derailleur for shifting a chain from one sprocket to another of a multiple chainwheel to perform thereby an intended speed change.
2. Description of the Prior Art
A bicycle front derailleur is mounted on the bicycle usually in combination with a rear derailleur which is used for shifting the chain from one sprocket to another of a multiple freewheel (rear gear). When the chain engages a larger chainwheel sprocket and a smaller freewheel sprocket, a greater gear ratio will result to enable one to ride the bicycle at high speed. Conversely, a combination of a smaller chainwheel sprocket with a larger freewheel sprocket gives a lower gear ratio, permitting one to ride at a lower speed but with a higher torque.
Generally, the front derailleur for bicycles includes a chain guide having an inner guide plate and an outer guide plate which are disposed at opposite sides of the chain at the position where the chain advances into engagement with the multiple chainwheel. The derailleur is adapted to translate the chain guide widthwise of the chain, i.e., laterally of the bicycle. A parallelogrammatic pantographic link mechanism is used for moving the chain guide in this way as disclosed, for example, in U.S. Pat. No. 4,674,995, FIG. 6. When the rider manipulates a shift lever on the bicycle frame, the pantographic link mechanism is deformed through a cable to thereby translate the chain guide laterally. As a result, the inner or outer guide plate of the chain guide pushes the chain laterally, whereby the chain in engagement with one sprocket of the chainwheel is shifted to another sprocket to change the speed of the bicycle.
With reference to FIG. 3, a more specific description will be given of the mechanism for shifting the chain guide of the conventional bicycle front derailleur.
The parallelogrammatic pantographic link mechanism, indicated at 10, comprises four members, i.e., a base member 2 fixed to the seat tube 1 of a bicycle by a clamp band, inner and outer link members 3, 4 pivoted to the base member 2 at their
5 upper base ends 3a, 4a by pins 5, 6, respectively, and a movable member 9 pivoted to the lower ends of the inner and outer link members 3, 4 by pins 7, 8, respectively. The four pins 5, 6, 7, 8 pivotally connecting the four members together are disposed at positions corresponding to the four corners of a parallelogram. Thus, when the link mechanism 10 deforms, the movable member 9 translates laterally inward or outward. The movable member 9 is integrally formed with a chain guide 11 comprising an inner guide plate 11a and an outer guide plate 11b. The inner guide plate 11a and the outer guide plate 11b are generally in parallel to each other and spaced apart by a distance permitting a chain C to pass therebetween.
The link mechanism 10 is always biased by a spring 12 so as to move the chain guide 11 laterally inward, i.e., toward a smaller sprocket Sa. The outer link member 4 is integrally provided at its upper end with an actuating arm 13 extending upward obliquely inward and having its free end connected to one end of a cable 14. The cable 14 extends along the frame and is connected to a shift lever so positioned as to be readily movable by the rider.
The shift lever is pivotally movable to pull or pay out the cable, thereby to move the cable longitudinally thereof.
Accordingly, when the rider manipulates the shift lever to pull the cable 14 in the direction of arrow P in FIG. 3, the link mechanism 10 deforms against the elastic force of a spring 12, shifting the chain guide 11 in the direction of arrow p, whereby the chain C, engaging the smaller sprocket Sa is pushed by the inner guide plate 11a into engagement with a larger sprocket Sb. Conversely, when the chain C engaging the larger sprocket Sb is to be shifted to the smaller sprocket Sa, the shift lever is pivoted in the opposite direction to pay out the cable 14. This causes the link mechanism 10 to restore itself by being biased by the spring 12 while being regulated by the payout amount of the cable 14 in the direction of arrow Q thereby returning the chain guide 11 in the direction of arrow q. At this time, the chain C is pushed by the outer guide plate 11binto engagement with the smaller sprocket Sa.
Thus, the conventional front derailleur shown in FIG. 3 is adapted to deform the link mechanism 10 by downwardly pulling the cable 14 attached to the free end of the actuating arm 13 which extends obliquely upward from the upper end of the outer link member 4. Accordingly, the derailleur has the following drawbacks.
First, the actuating arm 13 projects upward from the pantographic link mechanism 10, so that there is a hazardous likelihood that the garment of the rider will be caught by the arm 13 or the end of the cable 14 attached thereto.
Second, the actuating arm 13 is adapted to be pulled downward, so that the cable 14 connected thereto needs to extend downward to the bottom lug (crank axle support portion) of the bicycle frame and then to be greatly bent to extend along the downtube of the frame to the shift lever, hence a complex cable arrangement. This entails the following problems. Since the cable as greatly bent at an intermediate portion must be pulled longitudinally thereof, the cable encounters great resistance when pulled, is not smoothly movable and fatigues or breaks relatively easily. Because the cable extending from the front derailleur to the shift lever has a relatively large length, the longitudinal elastic elongation of the cable impairs the responsiveness of the derailleur to the manipulation of the shift lever. Further since the cable extends near the bottom lug, the wheel is liable to splash the cable with mud to result in the problem of accelerated deterioration of the cable due to corrosion or the like.